This invention relates to a color mixing system and method and more specifically to an RGB, light emitting diode controller for providing desired colors.
Conventional color control systems employ a feedback control arrangement to maintain a desired color emitted by for example an RGB, LED light source. However, it is known that visual sensitivity to small color differences is one of the considerations when determining the precision of a color control system.
Traditionally, in order to control and maintain a desired light color and intensity, a color space diagram is employed and various primary color light sources, such as Red, Green and Blue are controlled in accordance with the values represented by the color space diagram.
An exemplary color space is the RGB space, which is represented by a three-dimensional space whose components are the red, green, and blue intensities, along with their spectrum that make up a given color. For example, scanners read the amounts of red, green, and blue light that are reflected from an image and then convert those amounts into digital values. Displays receive the digital values and convert them into red, green, and blue light seen onscreen. RGB-based color spaces are the most commonly used color spaces in computer graphics, primarily because they are supported by many color displays and scanners. However, a shortcoming with using an RGB color space is that it is device dependent and additive.
Some color spaces can express color in a device-independent way. Whereas RGB colors vary with display and scanner characteristics, device-independent colors are meant to be true representations of colors as perceived by the human eye. These color representations, called device-independent color spaces, result from work carried out in 1931 by the Commission Internationale d""Eclairage (CIE) and for that reason they are also called CIE-based color spaces.
The CIE created a set of color spaces that specify color in terms of human perception. It then developed algorithms to derive three imaginary primary constituents of colorxe2x80x94X, Y, and Zxe2x80x94that can be combined at different levels to produce all the color the human eye can perceive. The resulting color model, CIE, and other CIE color models form the basis for all color management systems. Although the RGB and CMYK values differ from device to device, human perception of color remains consistent across devices. Colors can be specified in the CIE-based color spaces in a way that is independent of the characteristics of any particular display or reproduction device. The goal of this standard is for a given CIE-based color specification to produce consistent results on different devices, up to the limitations of each device.
There are several CIE-based color spaces, such as xyL, uvL, u*v*L, a*b*l, etc., but all are derived from the fundamental XYZ space. The XYZ space allows colors to be expressed as a mixture of three tristimulus values X, Y, and Z. The term tristimulus comes from the fact that color perception results from the retina of the eye responding to three types of stimuli. After experimentation, the CIE set up a hypothetical set of primaries, XYZ, that correspond to the way the eye""s retina behaves.
The CIE defined the primaries so that all visible light maps into a positive mixture of X, Y, and Z, and so that Y correlates approximately to the apparent lightness of a color. Generally, the mixtures of X, Y, and Z components used to describe a color are expressed as percentages ranging from 0 percent up to, in some cases, just over 100 percent. Other device-independent color spaces based on XYZ space are used primarily to relate some particular aspect of color or some perceptual color difference to XYZ values.
FIG. 1 is a plot of a chromaticity diagram as defined by CIE (Commission Internationale de l""Eclairage). Basically, the CIE chromaticity diagram of FIG. 1 illustrates information relating to a standard set of reference color stimuli, and a standard set of tristimulus values for them. Typically, the reference color stimuli are radiations of wavelength 700 nm for the red stimulus (R), 546.1 nm for the green stimulus (G) and 435.8 nm for the blue stimulus (B). Different color points along curve 60 can be combined to generate a white light depicted at point 62. The chromaticity diagram shows only the proportions of tristimulus values; hence bright and dim colors having the same proportions belong to the same point.
As mentioned before, one drawback of the XYZ space as employed for controlling an RGB light source is that in a system that is configured to control a desired color point, for example, Xw, Yw, Zw, a deviation from this desired color point may have a different visual impact, depending on the direction of the deviation. That is the perceptual color difference for the same amount of error in the color point location, would be different depending on where the color point with error is located, on the chromaticity diagram, in relation to the desired color point location.
Therefore, even if a system is employed with a very small error control scheme, the perceptual color difference may be still large for certain errors and excessively small for other color point errors. As such, the feedback system either over compensates or under compensates color point errors.
Thus, there is a need for an RGB LED controller system that employs a feedback control arrangement that substantially corrects all color point errors without visual perception of change in color.
In accordance with one embodiment of the invention, a control system for generating a desired light by a plurality of Red, Green and Blue light emitting diodes (LEDs) comprises a sensor responsive to a light generated by the LEDs to measure the color coordinates of the generated light, wherein the color coordinates are defined in an X, Y, Z color space. A transformation module is coupled to the sensor to transform the coordinates of the generated light to a second color space, such as an xxe2x80x2, yxe2x80x2 color space, in accordance with a Farnsworth transformation. A reference module is configured to provide reference color coordinates corresponding to the desired light, wherein the reference color coordinates are expressed in the second color space. An error module is coupled to the transformation module and the reference module and is configured to generate an error color coordinate corresponding to a difference between the desired white light color coordinates and the generated white light color coordinates. A driver module is coupled to the error module and is configured to generate a drive signal for driving the LEDs.